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Fuel pressure fluctuations on 75 M20F

75_M20F

By 75_M20F
in Vintage Mooneys (pre-J models)

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RLCarter Grand Master

RLCarter

Posted
Posted

What happens when you turn on the electric pump?... if it still moves around I would think it’s in the gauge

carusoam Grand Master

carusoam

Posted
Posted

Set the fuel gauge hose up again... a certain amount of air belongs in their... the air acts as a buffer...

I believe the procedure is in the MM...

@Bob_Belville was our go to guy to describe this affect...

PP thoughts only,

Best regards,

-a-

75_M20F Rising Star

75_M20F

Posted
  • Author
Posted (edited)

Also,

In addition to the pulsating as shown in the video above....

The fuel pressure gauge will never go to zero no matter what. This picture is with the engine shut down.....  

Time for a gauge overhaul? If so any recommendations who can repair or overhaul it?

 

 

 

 

 

Mooney_fuel_preuuse.thumb.jpg.2a73a45cd1ee5bd2c22c2525564cf5dc.jpg

Edited by 75_M20F
Immelman Mentor

Immelman

Posted
Posted

Well that doesn't seem right...

I had my combo MP/FP overhauled in 2007 at "Sky West Instruments", in Upland, CA. It was $300.

I imagine there are quite a few used versions of that gauge available now with so many people going to electronic primary instruments.

75_M20F Rising Star

75_M20F

Posted
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Posted (edited)
6 hours ago, RobertGary1 said:

It takes some time as I recall for pressure to bleed off. If you go fuel rich after shutdown will it go to zero?

No, it never reads zero no matter what. It has been removed from the airplane and still reads 8PSI. I am going to have it sent it off for repair.

Edited by 75_M20F
Marauder Grand Master

Marauder

Posted
Posted
No, it never reads zero no matter what. It has been removed from the airplane and still reads 8PSI. I am going to have it sent it off for repair.


I suspect you have some sort of obstruction in the fuel line going to the gauge or in the inlet of the gauge.



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75_M20F Rising Star

75_M20F

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Posted
15 hours ago, Marauder said:

 


I suspect you have some sort of obstruction in the fuel line going to the gauge or in the inlet of the gauge.



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The gauge has been shipped off for repair.

 

The braided hose that runs from the cockpit side of the firewall to the fuel pressure gauge...  how often is that hose supposed to be replaced?

Marauder Grand Master

Marauder

Posted
Posted
The gauge has been shipped off for repair.
 
The braided hose that runs from the cockpit side of the firewall to the fuel pressure gauge...  how often is that hose supposed to be replaced?


I don’t know if there is a recommended time limit but like the oil pressure line that comes into the cockpit to supply that gauge, if it has been in there for more than 10 years, I would not hesitate replacing it. I suspect many are in there for the lifetime of the plane.

I have a friend with a J model and the fuel pressure line actually leaked onto the feet of his passenger.


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PT20J Grand Master

PT20J

Posted
Posted

I’d replace it with a teflon hose that has no life limit. Might want to replace all the engine hoses if they are not teflon and are older than 5 years.

jetdriven Grand Master

jetdriven

Posted
Posted

I ripped out the factory mechanical oil pressure gauge, and  installed an electronic oil pressure gauge from the 1980 model, and then got the transducer as well, and wired it up.

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75_M20F Rising Star

75_M20F

Posted
  • Author
Posted (edited)

OK,  disconnected both ends of the line that runs from the fuel pressure gauge to the servo and blew it out with compressed air. Also removed and replaced the line from the firewall to the back of the gauge with a new hose.

Also, during this the gauge was stuck on 5PSI, so I sent it out to be overhauled also. The shop reported back that the bellows in the gauge had fuel in it.....

Unfortunately, the fuel pressure gauge is still pulsating just the same as before....

So what else can it be causing the pulsating?  Fuel pump?

Edited by 75_M20F
carusoam Grand Master

carusoam

Posted
Posted

Fuel pressure gauges get installed with a snubber...

Some times the gauge has an integral snubber...

The snubber acts like a muffler for the pressure noise that occurs naturally in the line...

Essentially it is a tiny calibrated hole that allows air to pass back and forth through...

Installing the gauge properly requires following a procedure to make sure the right amount of air is at the end of the tube...

Not too much or too little...

This topic comes up randomly when people either install or change a fuelP sensor...   Bob went through a bunch of trouble when he first installed a JPI930 in his ship (Gypsy Rose)...

The snubber is often referenced in the parts catalog... (?)

PP thoughts and memories of Bob’s posts...

Best regards,

-a-

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75_M20F Rising Star

75_M20F

Posted
  • Author
Posted
9 minutes ago, carusoam said:

Fuel pressure gauges get installed with a snubber...

Some times the gauge has an integral snubber...

The snubber acts like a muffler for the pressure noise that occurs naturally in the line...

Essentially it is a tiny calibrated hole that allows air to pass back and forth through...

Installing the gauge properly requires following a procedure to make sure the right amount of air is at the end of the tube...

Not too much or too little...

This topic comes up randomly when people either install or change a fuelP sensor...   Bob went through a bunch of trouble when he first installed a JPI930 in his ship (Gypsy Rose)...

The snubber is often referenced in the parts catalog... (?)

PP thoughts and memories of Bob’s posts...

Best regards,

-a-

Hello,

 

My bird has the older setup. A fuel line that runs from the pressure gauge out to the servo. I cleaned the entire line out and sent the gauge out for overhaul as it was stuck on 5 PSI. So, I don't know where this snubber would be?

 

carusoam Grand Master

carusoam

Posted
Posted

The snubber is often integral with the gauge... or mounted between the gauge and the pressure line...

IE as close as possible to the gauge...

Done electronically, an averaging function is often used to keep the numbers from flipping often...

Best regards,

-a-

Yetti Grand Master

Yetti

Posted
Posted
On 12/5/2020 at 8:31 AM, 75_M20F said:

The gauge has been shipped off for repair.

 

The braided hose that runs from the cockpit side of the firewall to the fuel pressure gauge...  how often is that hose supposed to be replaced?

It has 100LL fuel in it.  It is above your feet.  If it leaks where would it end up.    Braided stainless is the only answer.   Or better yet.  Wires and transducer that sits on the other side of the FireWall

 

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